[PATCH v2 5/5] powerpc/lib: inline memcmp() for small constant sizes

Christophe Leroy
2018-05-17T10:50:03Z

In my 8xx configuration, I get 208 calls to memcmp()
Within those 208 calls, about half of them have constant sizes,
46 have a size of 8, 17 have a size of 16, only a few have a
size over 16. Other fixed sizes are mostly 4, 6 and 10.

This patch inlines calls to memcmp() when size
is constant and lower than or equal to 16

In my 8xx configuration, this reduces the number of calls
to memcmp() from 208 to 123

The following table shows the number of TB timeticks to perform
a constant size memcmp() before and after the patch depending on
the size

Before After Improvement
01: 7577 5682 25%
02: 41668 5682 86%
03: 51137 13258 74%
04: 45455 5682 87%
05: 58713 13258 77%
06: 58712 13258 77%
07: 68183 20834 70%
08: 56819 15153 73%
09: 70077 28411 60%
10: 70077 28411 60%
11: 79546 35986 55%
12: 68182 28411 58%
13: 81440 35986 55%
14: 81440 39774 51%
15: 94697 43562 54%
16: 79546 37881 52%

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
arch/powerpc/include/asm/string.h | 46 +++++++++++++++++++++++++++++++++++++++
1 file changed, 46 insertions(+)

diff --git a/arch/powerpc/include/asm/string.h b/arch/powerpc/include/asm/string.h
index 35f1aaad9b50..80cf0f9605dd 100644
--- a/arch/powerpc/include/asm/string.h
+++ b/arch/powerpc/include/asm/string.h
@@ -4,6 +4,8 @@

#ifdef __KERNEL__

+#include <linux/kernel.h>
+
#define __HAVE_ARCH_STRNCPY
#define __HAVE_ARCH_STRNCMP
#define __HAVE_ARCH_MEMSET
@@ -51,10 +53,54 @@ static inline int strncmp(const char *p, const char *q, __kernel_size_t size)
return __strncmp(p, q, size);
}

+static inline int __memcmp1(const void *p, const void *q, int off)
+{
+ return *(u8*)(p + off) - *(u8*)(q + off);
+}
+
+static inline int __memcmp2(const void *p, const void *q, int off)
+{
+ return be16_to_cpu(*(u16*)(p + off)) - be16_to_cpu(*(u16*)(q + off));
+}
+
+static inline int __memcmp4(const void *p, const void *q, int off)
+{
+ return be32_to_cpu(*(u32*)(p + off)) - be32_to_cpu(*(u32*)(q + off));
+}
+
+static inline int __memcmp8(const void *p, const void *q, int off)
+{
+ s64 tmp = be64_to_cpu(*(u64*)(p + off)) - be64_to_cpu(*(u64*)(q + off));
+ return tmp >> 32 ? : (int)tmp;
+}
+
+static inline int __memcmp_cst(const void *p,const void *q,__kernel_size_t size)
+{
+ if (size == 1)
+ return __memcmp1(p, q, 0);
+ if (size == 2)
+ return __memcmp2(p, q, 0);
+ if (size == 3)
+ return __memcmp2(p, q, 0) ? : __memcmp1(p, q, 2);
+ if (size == 4)
+ return __memcmp4(p, q, 0);
+ if (size == 5)
+ return __memcmp4(p, q, 0) ? : __memcmp1(p, q, 4);
+ if (size == 6)
+ return __memcmp4(p, q, 0) ? : __memcmp2(p, q, 4);
+ if (size == 7)
+ return __memcmp4(p, q, 0) ? : __memcmp2(p, q, 4) ? : __memcmp1(p, q, 6);
+ return __memcmp8(p, q, 0);
+}
+
static inline int memcmp(const void *p,const void *q,__kernel_size_t size)
{
if (unlikely(!size))
return 0;
+ if (__builtin_constant_p(size) && size <= 8)
+ return __memcmp_cst(p, q, size);
+ if (__builtin_constant_p(size) && size <= 16)
+ return __memcmp8(p, q, 0) ? : __memcmp_cst(p + 8, q + 8, size - 8);
return __memcmp(p, q, size);
}

--
2.13.3